It is known that certain metals, commonly referred to as "shape-memory alloys", undergo a temperature related phase change which is characterized by the memory of any mechanical configuration imposed on the material at the annealing temperature T.sub.a. When the material is below some lower temperature T.sub.o, it possesses a particular crystal structure whereby the material may be deformed into an arbitrary shape with relative ease. Upon heating the shape-memory alloy above a higher temperature T.sub.e, it undergoes a change in crystal structure and the memory effect is manifested by a resumption of the originally imparted shape, representing the onset of a restoring stress, where T.sub.o &lt;T.sub.e &lt;T.sub.a. The transition temperature range of a shape-memory alloy, over which the phase transformation occurs, is defined as being between T.sub.o and T.sub.e. These memory materials have been produced in bulk form primarily in the shape of wires, rods, and plates.
The best known and most readily available memory alloy is Nitinol, an alloy of nickel and titanium. With a temperature change of as little as 18.degree. F., Nitinol can exert a force of as much as 60,000 psi when exerted against a resistance to changing its shape.
Actuators have heretofore been developed which employ shape-memory alloys or materials. These actuators generally operate on the principal of deforming the shape-memory alloy while it is below its phase transformation temperature range and then heating it to above its transformation temperature range to recover all or part of the deformation, and in the process create motion of one or more mechanical elements. The actuators employ one or more shape-memory effect elements produced in bulk form and are therefore limited in size and usefulness. U.S. Pat. Nos. 4,055,955 3,725,835, 4,553,393 and 3,849,756 disclose such actuators employing bulk form shape-memory elements
Micro-actuators a few micro meters in size are presently needed for opening and closing valves, moving switches and generally providing motion for micro-mechanical devices. The means by which motion may be created in such small dimensions are very limited and subject to problems. For example, actuator have often employed means utilizing electrostatic forces. However, electric fields exert a force which is proportional to the area of the conductor. Thus, as the dimensions of the actuator decrease the electrostatic forces also decrease.
The disclosed invention employs the unique properties of shape-memory alloys to micro-actuators by means of thin film technology. The use of shape-memory alloys in micro-actuators will increase the performance of actuators for micro-mechanical devices by several orders of magnitude. This is accomplished by the fact that both stress and strain of the shape memory effect can be very large, providing substantial work output per unit volume. For example, the work output of the nickel-titanium shaped memory alloy is of the order of 1 joule per gram per cycle. Correspondingly, a shape memory film micro-actuator 1mm on each side and 10 microns thick (volume equals 10.sup.-6 cm.sub.3 weights approximately 64 micro-grams and can be expected to produce 64 microjoules of work per cycle. By contrast, an electrostatic actuator of the same volume might generate 10.sup.-3 microjoule and a similar piezo-electric device is limited to about 0.1 microjoule per cycle, depending on voltage.
Micro-mechanical actuators of shape-memory film will also provide the following advantages: (1) exert stresses of hundreds of mega-pascals; (2) tolerate strains of more than 3%; (3) work at common TTL voltages, these being much lower than electrostatic or piezo requirements; (4) be directly powered with electrical leads on a chip; and (5) survive millions of cycles without fatigue.
As previously discussed, the disclosed invention employs thin films of shape-memory alloy to produce actuators of micron size. The disclosed method for depositing these shape-memory films results in a material cystallography which is mostly a disordered structure with a small amount of included ordered precipitates; this being evident when the material is in its high temperature phase. However, the general expectation of shape-memory alloys in the field of crystallography is that an ordered structure is required to manifest the shape-memory effect. The disclosed invention teaches that this is not the case and that the shape-memory effect which results from a partially disordered crystal structure may in fact be better than the effect in conventional material.
Further, it is not readily apparent that a thin film of a material which exhibits shape-memory characteristics in bulk form will exhibit shape-memory characteristics in bending. A thin film is, because of its high aspect ratio, basically a two dimensional body. Any shape-memory characteristics exhibited in bending would, accordingly, be expected to be significantly attenuated if present at all. The disclosed invention teaches that not only does thin film shape-memory alloy exhibit a strong shape-memory recovery in bending, but it does so with less than 0.5% strain which is not at all characteristic of conventional bulk shape-memory alloy. The conventional shape-memory alloy must typically be strained in excess of 1%.
A Japanese group comprising Minemura, et al. at the Hitachi Research Laboratory in Japan has reported shape-memory effect in sputter deposited films of cooper-aluminum-nickel (Journal of Material Science Letters 4 (1985) 793-796). The shape-memory allegedly achieved, however, was in connection with composite materials of cooper-aluminum-nickel films on aluminum foil. The composite material was bent in liquid nitrogen and returned to its original shape as the temperature rose. Since this property was not established in the film by itself, it is suspected that the observed phenomenon was a result of differential thermal expansion stemming from two dissimilar materials in intimate contact. Moreover, a gradual return to the original shape as observed by the authors does not suggest the typical shape-memory transition, which tends to occur suddenly at the transition temperature.
A general object of the invention is to provide an improved actuator device suitable for micro-mechanical applications.
It is another object of the present invention to provide a shape-memory alloy micro-actuator device employing an actuating element that is less than 10 microns thick, and capable of creating motion of mechanical elements a few micro-meters in size.
Another object is to provide a method for producing a thin film of shape-memory alloy to provide such a force on or motion of a micro-mechanical element.
Another object is to provide a two-state micro-actuator employing a pair of micro-actuating elements comprising thin films of shape-memory alloy, and a method for producing same.